A mixture in a state where crude oil is mixed with salt water is mined in an oilfield area, and oilfield produced water is formed by separating crude oil and salt water from this mixture. Since the oil field produced water contains a considerable amount of oil droplets and dissolved organic compounds, there is a fear that returning it to the sea, the lakes and marshes will bring about the destruction of the environment.
Conventionally, various methods have been discussed to separate and remove the oil contained in oilfield produced water which is produced in an oilfield area. For instance, in JP-A No. 2003-144805, it is disclosed that the oil contained in oilfield produced water is made into an emulsion, and a magnetic floc is form by adding magnetic powder and a coagulant therein, and the magnetic floc is separated by magnetic aspiration.
Moreover, “Experience of Produced Water Treatment in the North Sea, Marine Pollution Bulletin, Vol. 29, No. 6-12, (1994), p. 312-316” describes that oil droplets are separated by using a liquid cyclone.
However, even when oil droplets in the oilfield produced water are removed, a considerable amount of organic compounds, for instance, acetic acid, propionic acid, and valeric acid, etc. are dissolved in the oilfield produced water and they become a reason for the bad small and the damage to the environment, so that a removal technique has been discussed. The index to measure the content of the aforementioned dissolved organic compound is a value called the COD (chemical oxygen demand). Therefore, the dissolved organic compound associated with the aforementioned oilfield goes by the name of COD element.
In JP-A No. 2004-275884, it is described that activated carbon powder is added to the oilfield produced water to absorb the COD elements and the activated carbon powder congregates and is separated by adding a coagulant.
In JP-A No. 2004-255290, it is described that an organic polymer coagulant is added to the oilfield produced water and, moreover, the oil content is decomposed by adding a polymer coagulant and an oxidant. It is also described that excess oxidant is absorbed and removed by using activated carbon, and that hypochlorous acid, etc. formed by electrodialysis is used for an oxidant.
The technologies described in the various aforementioned well-known documents have a problem from the viewpoint of suitable treatment of the oil content in the oilfield produced water and the dissolved organic compounds which will be a problem hereafter and treatment of the separated organic compound. For instance, a large amount of sediment becomes a serious problem in a method for precipitating the activated carbon, on which an organic material is adsorbed, by using a coagulant. Though the oil content may be decomposed to a certain point by using an oxidant such as hypochlorous acid, etc., it is not clear that the dissolved organic compound can be effectively decreased.
In JP-A NO. 2000-93957, a technology is disclosed in which water including a small amount of organic material, such as drinking water, etc. is purified by using a water purification film which contains a zeolite. However, it is not a technology which processes very poor water which contains a large amount of oil and organic compounds such as the oilfield produced water.
[Patent Document 1] JP-A No. 2003-144805
[Patent Document 2] JP-A No. 2004-275884
[Patent Document 3] JP-A No. 2004-255290
[Patent Document 4] JP-No. 2000-093957
[Non-patent Documents 1] Experience of Produced Water Treatment in the North Sea, Marine Pollution Bulletin, Vol.29, No. 6-12, (1994), p. 312-316